C-3 alkylation of indoles in general is a facile process, as shown in Comprehensive Organic Chemistry, 1979, V. 4, Chapter 17, “Indoles and Related Systems”. This alkylation can occur under basic or acidic reaction conditions. The effect of different metal cations, base concentration, and catalysts for the phase transfer alkylation of indoles under basic conditions has been studied. Alkylation of indoles under acidic reaction conditions can often proceed with catalytic amounts of acid catalysts. An examination of the literature reveals a large number of acid catalysts that have been used for the C-3 alkylation of indoles.
It is convenient to achieve both alkylation and reduction transformations simultaneously when indoles with C-3 saturated substituents are desired. Triethylsilane is a convenient reducing agent under acidic reaction conditions, and trifluoroacetic acid and triethylsilane have been used as a reagent combination to accomplish simultaneous alkylation and reduction at the C-3 position of indoles; Steele, et al. Tet Lett 1993, p 1529. Unfortunately, trifluoroacetic acid is incompatible with some acid sensitive functional groups, and, in fact, trifluoroacetic acid is well known as a reagent for the cleavage of benzhydryl protecting groups; Greene and Wuts, “Protective Groups in Organic Synthesis” 3rd ed., John Wiley & Sons, 1999.